The International Publication WO 2005/088726 discloses a technique of using a transparent amorphous oxide semiconductor film containing indium, gallium, zinc, and oxygen as the channel layer (also called active layer) of a thin-film transistor (TFT). The transparent amorphous oxide semiconductor film can be formed at a low temperature and is transparent to visible light, and thereby a flexible transparent TFT can be formed on a substrate such as a plastic plate or a plastic film.
Nature, vol. 432, pp. 488-492 (2004) discloses that a transparent amorphous oxide semiconductor film having a composition ratio of In:Ga:Zn being 1.1:1.1:0.9 according to an X-ray fluorescence method has a transmittance of approximately 80% or more to visible light. It also describes that a transparent TFT can be formed. According to Japanese Patent Application Laid-Open No. 2002-76356, developments are actively made in a TFT using, as the channel layer, a transparent conductive oxide polycrystalline thin-film containing ZnO as a main ingredient.
In recent years, a light-emitting device utilizing organic electroluminescence (EL) has been actively researched and developed. When the organic EL element is applied to a display, active matrix driving is typically performed. For the active matrix driving, a TFT made of amorphous silicon or polycrystalline silicon is normally employed.
There is proposed an active matrix organic EL display for the purpose of providing a high-quality and high-luminance display capable of preventing deterioration of a driving element and also capable of performing television video display. Japanese Patent Application Laid-Open No. H09-114398 discloses that an organic EL display using two MOS field effect transistors each of which includes an active layer made of single crystalline silicon obtains high quality and prevents the deterioration of the driving element.
As a result of studies made by the present inventors in order to manufacture a transparent field effect transistor using a transparent amorphous oxide semiconductor film, the present inventors have found a phenomenon in which electric conductivity changes under visible light having a specific wavelength.
In order to examine the phenomenon in details, the present inventors performed experiments for measuring electric conductivity during spectral light irradiation (spectral sensitivity measurement experiments) as described later. As a result, the present inventors have found that the electric conductivity changes (increases) in a short-wavelength range side of visible light because of light absorption (see FIG. 9).
FIG. 9 is a graph showing the plotting of drain currents at every wavelengths of irradiated spectral light at the time of on-operation and at the time of off-operation of a thin-film transistor (TFT) when the TFT is irradiated with every 10 nm at the wavelengths of spectrum of visible light having a constant strength. In the case of irradiation with visible light, the off-current of the TFT is significantly changed at the time of irradiation of light, particularly on a short-wavelength side. This change affects the stable operation of the TFT. That is, the present inventors have found for the first time that, in an actual transparent amorphous oxide which is considered as being transparent to visible light, a change in electric conductivity due to light excitation, that is, the generation of photo carriers actually occurs in a visible light range.
Further, the present inventors have found a problem that, even in a case where a material which is normally considered as being transparent oxide (that is, oxide which does not absorb light) is used for the active layer of the TFT, when the TFT and the light-emitting device are combined with each other for operation, the operation of the TFT becomes unstable because the TFT absorbs a part of short-wavelength light emitted from the light-emitting device.
In order to prevent the light from affecting the characteristics of such element, a light shielding unit such as a light shielding film is normally provided. However, when the light shielding unit is provided, the degree of freedom of design in a device structure significantly reduces which utilizes the transparency of the active layer, that is, the active layer itself of the TFT as a window layer.